Linear magnetron traveling wave tube



July 21, 1959 c. K. BIRDSALL ET AL 2,396,117

LINEAR MAGNETRON TRAVELING WAVE TUBE Filed Feb. 28, 1955 Q- r r Ara/z; 6/021: 445/1011,

United States Paten MAGNE'DRON TRAVELING VVAVE TUBE Charles K. "Birdsall and Lester-M..Field, Los.Angeles,

Califi, assign'ors to Hughes Aircraft Company, Culver City, Calif., a corporation'of Delaware Application February 28; -1955;-Serial N0149IL953 L 3 Claims. (Cl. 315 -393) stream is subjected to a unidirectional..-transversez'magnetic field. Such an amplifier is described'in US. Patent Number 2,607,904'toLerbs. The:slow'-wave' structure of theamplifier therein described is maintained at a potential positive with respect -.to-that oftzthe conductive plate. The stream is introduced between the plate andzthe slowwave structureat an appropriate idistance'from'the: plate .and with.. aisuitable :axial velocity. Theiistream. then follows a path t which :is inclined slightly. tov'varda. the

slow-wave structure. When an. electitomiag'netic wave is launched alongthe .slow-waveastructure; 'the :stream interacts with .thewave to 'causezit togrow orbe amplified. The stream thus loses axial: momentum by giving up energy tothe wave. vThe restraining force of the transverse magnetic. field is decreased by thedecrease in axial stream velocity and the'r'transverse electrostatic field causes the. stream to bed-irected slightly toward the slow-wave structure.

The gain andefiiciency and the radio-frequency or RF power output oftraveling-wave magnetrons employing conventional .conductive .shelices'.arerestricted because an electron stream. is vpr'ojected'only near aportion of the outside of a conductive helix. Thelongitudinal field is shorted outat the conductive" plate. and greatly reduced due vto the :presenceeoflthe plate in the region betweenv the helix or "other.tslow wave circuitandthe plate. Hence, the. avera'ge'flongitudinal':field is seriously reduced .and gain .and efiiciency of thetube'is-low.

. It is thereforeanobjectofthe-invention"to-provide an improved slow-wave structure .for. a traveling-wave .tube.

It is. another object of :.thei.invention to' provideemeans for increasing the 'elficiencyiand.rradio-frequency power output of traveling-wave magnetrons.

In accordance with the invention, a. slow-'wave' structure which may be split along-an .axial planerisempl'oyed to provide the electrical equivalent of acontr-awound conductive helix. Different potentials are applied between the separate halves of the'slow-w'av structure and an electron stream is projected between them. An electromagnetic wave is launched-along thesloW-waver'structure, and the stream is permitted to..-interact withl'the wave. Theamplification produced is thus increasedover that produced in conventional traveling-wave magnetrons because interaction between the wave and the stream is possible in substantially all of the space occupied by the stream between the separate halves'of theslow-wave structure.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and ice advantages- 'thereof, will be'bette'r understood from the .-followingdescription considered in connection with the accompanying drawing in which an embodiment of the -inv'ention isillustrated by way of example. It is to be expressly understood;however, that thedrawing is for the-"purpose of illustration and description only, and is not intended as a definition of the limits of the inven-' tion.

Fig. 1 is a sectional view of a traveling-wave magne- "tron in whicha-slow-wave structure having the configuration bf cont-rawound conductive helices is shown;

'Fig. '2'-is anisometric view of the slow-wave'structure 'shown in'Fig; 1 at the leftend of the tube; and

- Fig. 3 is the"trajectory plot of an electron stream employed in' the traveling-Wave-magnetron of Fig. 1.

. Referringto the drawing, and-particularly to Fig. 1

thereis shown a traveling-wave magnetron 10 having a rectangular input waveguide 12 at its left end and a rectangular outputwaveguide 14 at its right end. The traveling-wave magnetron 10 comprises an evacuated envelope-ldwhich has an enlarged portion 18 at its left end for-hou'sing an electron gun 20. The gun 20 comprises a cathode 22 which is provided with a filament 24, a'focusing electrode'26, and a combined cathode support and focusing electrode 28. The filament 24 is heated by' means of a battery30. Cathode 22 is maintained a 'few 'hun'dred volts negative with respect to ground 'by"-means 'of an accelerating source'of potential 32. Focusing electrode 26-ismaintained at ground po- "tentiali' whereas focusing electrode 28 is maintained at the' cathcde'potential. The gun 20 is similar to one described *inthe Lerbspatent above referred to.

To the'iright of focusing electrodes 26 and 28, a slow 'wave'fstructure is shown inside the envelope 16 comprising'twmserpentine, conductive structures 34 and 36 whichsare similar to a'slow-wave structure disclosed and claimedin ace-pending application, Serial No. 456,682, entitled Traveling Wave Tube, filed'September 17, 1954, by Charles 'K; Birdsall and Lester M. Field, now abandoned. A. collector electrode 38 is disposed at the-right endiof-the. envelope 16 to intercept the stream electrons produced by'the gun--20. Collector electrode 38 is for thiszpurposemaintained 'at ground potential. A trans- .verser'magnetic field is produced-along the path of the stream produced by the gun 20 by' means of two magnetic solenoids 4tl, only one of which is shown in the sectional view.- of Fig. l. The solenoid 40 shown is provided with a source. of potential 42.

The' islow-wave structure of the tube 10 in Fig. l'is shown in a brokenisometric-View in Fig. 2. The-serpentine structures 34 and 36 are transverse images, that is, they zareoidenticalwexcept for thefact that-the right-hand portion zofrone is: mechanically equivalent tothelefthandtporti on of? the other. Each serpentine structure 34 and 36::coi'nprises a plurality of transversely extending conductive segments which are connected by axially extending; conductive :segments i122. The conductive segments -120-'may: have :semicircular shape although their specifierectangular:shape.shownumay be found .to be-more. eflicient and convenient toxuse. Both ofthese shapeswmaynbe generically classified as arches. Alternate. adjacent .pairs of -.the arched, conductive segments 12'0..are connected alternately to diametrically opposed points by, the axial segments 122. The axial segments 122 of each of theserpentine .structures 34 and 36 are then disposed contiguous to. one another .whereby. an electromagnetic coupling may be produced by the capacitance between the axial conductive segments 122 and the opposed ends of the arched conductive segments 120 respectively of each of the serpentine structures 34 and 36, which are disposed opposite one another.

As explained in the aforementioned Birdsall and Field application, the slow-wave structure including the serpentine structures 34 and 36 appears to be a contrawound conductive helix electrically. This is true because the segments 122 in both of the serpentine structures 34 and 36 are connected for purposes of wave propagation along the serpentine structures 34 and 36. It isto be understood that the number of pairs of the serpentine structures 34 and 36 is only limited by their practical mechanical strength. 'More than two may be employed although two may be found to be most convenient. Such structures are further described in the aforementioned Birdsall and Field application. V

In the operation of the traveling-wave magnetron in Fig. 1, a relatively thin electron sheet beam'is pr jected through the serpentine structures 34 and 36 by means of the electron gun 20. The transverse magnetic field produced by the solenoid 40 prevents electrons from traveling beyond a predetermined distance towards the serpentine structure 34 which is maintained at the same potential as focusing electrode 26, that is ground, whereas the serpentine structure 36 is maintained at the potential of the focusing electrode 28, which is the cathode potential, by an appropriate connection thereto. Serpentine structure 34 is thus maintained at a potential positive with respect to that of the serpentine structure 36. If the sheet beam produced by the gun 20 is introduced at a proper distance between the serpentine structures 34 and 36 it will travel along a stubstantially linear path as indicated by an arrow 130 in Fig. 3, where the line 132 represents the symmetrical axis of the slow-wave structure comprising the serpentine structures 34 and 36. The dashed lines 134 and 136 represent the transverse portions of the serpentine structures '34 and 36, serpentine structure 34 being maintained at a potentialfipositive with respect to that of serpentine structure 36 as' indicated by the and signs. As the beam moves along the path indicated by the arrow 130, the beam will give up energy to the wave propagated along the serpentine structures 34 and 36 launched therealong at the input end of the tube by the excitation of the input waveguide 12 of the tube 10. As interaction takes place, the stream electrons moving along the path 130 will lose axial velocity or momentum. Hence, the transverse electrostatic field existing between the serpentine structures 34 and 36 will pull the electrons towards the serpentine structure 34. The magnetic field produced .by the solenoid 40 will not prevent this movement because the axial velocity of electrons moving along the path 130 will be reduced and hence will reduce the magnetic force thereon restraining their movement towards the serpentine structure 34. A transverse magnetic field is indicated in both Fig. 3 and Fig. 1 by the crosses 140, the magnetic field being taken as having a direction into the paper of the drawing.

A traveling-wave magnetron of increased power and efiiciency may thus be constructed by practicing the present invention in that interaction between the electron stream and the traveling-wave may be increased.

What is claimed is:

1. A slow-wave structure for use in a linear magnetron type traveling-wave tube comprising a plurality of arched conductive segments axially aligned with each other about a predetermined path; a plurality of axial conductors, individual ones thereof being connected between adjacent segments, said axial conductors alternating be tween opposite sides of said path, whereby said arched segments and said axial conductors define a serpentine shaped element; said structure defining an opposed pair of said elements, said opposed pair of elements being separated in a direction perpendicular to said path and each element being the mirror image of the other, said elements being electromagnetically coupled together as a radio frequency unitary structure due to the close spacing between said elements; means for maintaining said elements at different directcurrent potentials and means for providing a magnetic field in the region of said slowwave structure .parallelto the plane of separation.

2. A slow-wave structure for use in a linear magnetron type of travelling-wave tube comprising a series of arched conductive segments axially aligned along the axis of the tube; a plurality of substantially straight axial segments disposed in an axial plane interconnecting adjacent arched segments, said axial segments alternating between opposite sides of said arched segments along the length of the tube, whereby said arched segments and said axial segments define a serpentine element; said slow-wave structure defining an opposed pair of said elements, said elements being separated in a direction perpendicular to said axial plane and each element being a direct current isolated mirror image of the other, means for maintaining said elements at different direct-current potentials, whereby an electrical field is caused to exist perpendicularly to said axial plane; and means for maintaining a magnetic field in the region of said tube parallel to said axial plane and transverse to the axis of said tube, said slow-wave structure being, as regards radio frequency energy, a unitary contrawound slow-wave structure.

3. A linear magnetron traveling-wave tube comprising a slow-wave structure having a first set of arched conductors disposed in the same position sequentially along a predetermined axis; a second set of arched conductors disposed sequentially along saidvaxis and having their extremities disposed in an opposed adjacent position to the extremities of said first set of arched conductors; a first set of straight axial segments interconnecting adjacent arched conductors of said first set of conductors, said first set of axial segments alternating between opposite extremities of said first set of conductors; a second set of straight axial segments disposed adjacent to said first set of straight axial segments and interconnecting adjacent arched conductors of said second set of conductors, said second set of axial conductors alternating between opposite extremities of said second set of conductors, whereby said first set of arched conductors and axial segments defines a first serpentine shaped element and said second set of arched conductors and axial segments defines. a second serpentine shaped element which is a mirror image of said first element, said first and second elements being direct current mutually insulated but radio frequency capacitively coupled, whereby the electrical equivalent of a contrawound helix is simulated, being made up of two continuous serpentine elements lying adjacent to each other and having axial plane of symmetry; means for maintaining said serpentine elements at different direct current potentials to provide an electric field perpendicular to said plane of symmetry and means for providing a magnetic field along said plane of symmetry in the region of said slow-wave structure transversely to the axis of said structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,687,777 Warnecke et al. Aug. 31, 1954 2,730,678 Dohler et a1 Jan. 10, 1956 2,768,322 Fletcher Oct. 23, 1956 2,813,221 Peter Nov. 12, 1957 FOREIGN PATENTS 699,890 Great Britain Nov. 18, 1953 

